Pneumatic rubber tires conventionally have rubber sidewalls composed of carbon black reinforced rubber compositions which are, therefore, black in color. Sometimes it is desirable to provide a decorative white colored rubber composition on a portion of an outer surface of such tire sidewall. Such rubber composition is conventionally colored white with titanium dioxide pigment and is devoid of, or does not contain, carbon black.
Sidewalls of rubber tires may age somewhat prematurely due to various factors which include (1) weathering due to atmospheric conditions such as, for example, exposure to ultraviolet light, ozone and high humidity; (2) fatigue cracking due to continual flexing of the tire sidewall under operating conditions; and (3) abrasion due to scuffing against road curbs and other objects. White colored tire sidewalls are of special concern because, due to the white color of the sidewalls, the weathering, fatigue cracking and abrasion may be more cosmetically observable than for black colored sidewalls.
In particular, it is well known that ozone causes surface cracking of conventional highly unsaturated rubber vulcanizates used in tires when the rubber is placed under strain in an ozone environment. The most severe deterioration occurs when a number of cracks are formed which grow rapidly into deep, disruptive fissures. These ozone cracks seriously shorten the serviceable life of the tire, especially in the area of the sidewall.
Conventional chemical antiozonants have been developed which retard the formation of ozone cracks occurring under static and dynamic conditions. Examples of antiozonants in common use include: N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine; N-phenyl-N'-isopropyl-p-phenylenediamine; N-phenyl-N'(-1,4-dimethylpentyl-p-phenylene diamine; N-phenyl-N'-(1-methylheptyl)-p-phenylenediamine; N-phenyl-N'-cyclohexyl-p-phenylenediamine; mixed diaryl-p-phenylene diamines; N,N'-diphenyl-p-phenylenediamine; N,N'-dibeta-naphthyl-p-phenylenediamine; N,N'-bis(1,4-dimethylpentyl)-p-phenylenediamine; N,N'-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine; N,N'-bis(1-methylheptyl)-p-phenylenediamine; N-phenyl-N'-p-toluenesulfonyl-p-phenylenediamine and blends of these materials.
While the use of these well-known paraphenylenediamine materials has improved the ozone protection under both static and dynamic conditions, the use of these materials adds to the cost of the rubber and even the best of the above class of compounds just described has a very strong tendency to both stain and discolor the rubber compound containing these materials.
In practice, for white sidewalls, weathering is typically retarded by the use of non-staining antidegradants and the use of low unsaturation rubbers such as, for example, halobutyl rubber--particularly chlorobutyl rubber, and EPDM rubber. EPDM rubbers are well known to those skilled in such art and, generally mean ethylene/propylene terpolymer elastomers, with a minor amount of non-conjugated diene (e.g., 1 to 15 percent of the terpolymer), e.g., hexadiene, dicyclopentadiene or ethylidene norbornene. The unsaturated part of the polymer molecule is pendant from the main chain, which is essentially completely saturated.